1. Field of the Invention
The present invention relates to a surface emitting laser, a method for producing a surface emitting laser, and an image forming apparatus.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) is a surface emitting laser capable of emitting light in a direction perpendicular to a substrate surface and has a feature that a two-dimensional array can be easily formed.
Densification and high-speed operation can be realized by the parallel processing of a plurality of beams emitted from the two-dimensional array, and various industrial applications such as optical communication are expected. For example, when a surface emitting laser array is used as a light source for exposure of an electrophotographic printer, densification and high-speed operation can be achieved in a step of forming an image using a plurality of beams.
In such electrophotographic application, it is necessary to stably form very small laser spots on a photosensitive drum. Therefore, a stable operation in a single transverse mode or a single longitudinal mode is one of the required laser characteristics of a VCSEL.
In surface emitting lasers such as the above-referenced VCSEL, a method has been developed in which a current confinement structure is formed using a selective oxidation technique, so that current can be selectively injected into a region necessary to realize high performance.
In this method, an AlGaAs layer (e.g., Al0.98Ga0.02As) having a high aluminum (Al) composition ratio is provided in a multilayer mirror; and this AlGaAs layer is selectively oxidized in a high-temperature water vapor atmosphere to form the current confinement structure. Since the oxidized region is changed from an electrically conductive region to an insulating region, current can be selectively injected into a desired position of an active layer region.
In order to achieve a high output in such a selective oxidation-type VCSEL, it is necessary to increase the diameter of an aperture serving as an electrically conductive region of the current confinement structure. However, the distribution of carriers, which carry the current, is concentrated on an edge portion of the aperture, the edge portion being a boundary between the electrically conductive region and an insulating region. Accordingly, when the diameter of the aperture is increased, a higher-order transverse mode, which has a large light intensity distribution in the edge portion, tends to oscillate.
To solve this problem, H. J. Unold et al., in “Large-Area Single-Mode Selectively Oxidized VCSELs: Approaches and Experimental”, Proceedings of SPIE Photon, West, Vol. 3946, (2000), pp. 207-218, (hereafter “Non-Patent Document 1”), discloses a method in which two current confinement structures are used. FIG. 10(b) of Non-Patent Document 1 has been reproduced as FIG. 12 herein.
In the method of Non-Patent Document 1, in which two current confinement structures are used, a current confinement structure 1230 having an aperture diameter smaller than the aperture diameter of another current confinement structure 1220 which is disposed near an active layer 1210 is arranged at the side that is away from the active layer. With this structure, carriers are concentrated on the central portion of the aperture in the current confinement structure 1220 disposed nearer to the active layer 1210. The current confinement structure 1220 disposed nearer to the active layer 1210 dominates the mode of resonant light. Accordingly, when the carriers are injected into the central portion of the aperture, the coupling efficiency between the carriers and fundamental mode light can be increased. Similarly, when two current confinement structures are used, oscillation in a higher-order mode can be suppressed and a high-output surface emitting laser can be obtained, as compared with the case where one current confinement structure is used.
To achieve a single transverse mode, it is necessary to perform an effective coupling between carriers and the fundamental mode light. For this purpose, in the technology disclosed in Non-Patent Document 1 in which two current confinement structures are provided, it is necessary that the aperture diameter of the current confinement structure 1230 disposed at the side that is away from the active layer 1210 be smaller than the aperture diameter of the current confinement structure 1220 disposed at the side near the active layer 1210.
For example, when the aperture diameter of the current confinement structure 1220 disposed at the side near the active layer 1210 is in the range of 6 to 7 μm, the aperture diameter of the current confinement structure 1230 disposed at the side that is away from the active layer 1210 is about a half of that of the current confinement structure 1220, i.e., in the range of about 3 to 4 μ.
In the normal mesa structure described in Non-Patent Document 1, in preparation of an oxidation current confinement structure having a small diameter, it is necessary to perform oxidation from a sidewall of the mesa over a long distance in a transverse direction. In contrast, the preparation of an oxidation current confinement structure having a large diameter requires oxidation from the sidewall of the mesa, but only over a short distance in the transverse direction.
However, according to empirical facts, with an increase in the oxidation distance, it becomes difficult to control the oxidation process and the probability in which the oxidation distance does not match with a design value increases, resulting in a decrease in the yield. In addition, when the oxidation distance increases, the resulting oxidized layer tends to be detached from adjacent semiconductor layers during an annealing treatment or during current injection, resulting in a decrease in the reliability of the resulting device.